The present invention relates to a combustion apparatus for NOx reduction to be applied to water-tube boilers, reheaters of absorption refrigerators, or the like.
Generally, as the principle of suppression of NOx generation, there have been known (1) suppressing the temperature of flame (combustion gas), (2) reduction of residence time of high-temperature combustion gas, and (3) lowering the oxygen partial pressure. Then, various NOx reduction techniques to which these principles are applied are available. Examples that have been proposed and developed into practical use include the two-stage combustion method, the thick and thin fuel combustion method, the exhaust gas recirculate combustion method, the water addition combustion method, the steam jet combustion method, the flame cooling combustion method with water-tube groups, and the like.
With the progress of times, NOx generation sources even of relatively small capacity such as water-tube boilers have been coming under increasingly stricter regulation of exhaust gas, and so further reduction of NOx is demanded therefor. The present applicant proposed NOx reduction techniques for these demands by Japanese Patent Laid-Open Publication HEI 11-132404 (Specification of U.S. Pat. No. 6,029,614).
This prior art technique is intended to achieve NOx reduction by a combination of cooling of burning-reaction ongoing gas with water tubes and cooling of burning-reaction ongoing gas with exhaust gas recirculation. However, the technique was capable of NOx reduction up to only about 25 ppm, other than one that allows NOx reduction to below 10 ppm to be achieved. It is noted that NOx reduction with the value of NOx generation being not more than 10 ppm will hereinafter be referred to as super NOx reduction.
Also, from recent years"" regulations of NOx value, there has been an increasing demand for achieving regulation values not only at some operating points but also over a wider range of operating points, i.e., those throughout a day or a year. The above-mentioned prior art technique was not one that could meet this demand.
Further, the present applicant, through continued studies on the super NOx reduction technique of steam boilers in response to the society""s demand, has reached a practicalization of a super NOx reduction technique with steam boilers. During the process of studies on this super NOx reduction technique, the applicant found out that in order to realize a stable super NOx reduction, controlling the excess air ratio to a constant one is of importance and the greatest factor of variations in excess air ratio is outside-air temperature.
An object of the present invention is to provide a combustion apparatus for NOx reduction capable of achieving stable NOx reduction with simple means.
The present invention having been accomplished to solve the above object, the invention provides a combustion apparatus for NOx reduction by suppressing temperature of combustion gas derived from a burner, comprising: NOx reduction means having an excess air ratio versus NOx characteristic (NOx emission characteristic) that generated NOx value decreases with increasing excess air ratio of the burner, and an excess air ratio versus CO characteristic that exhaust CO value increases with increasing excess air ratio; and excess-air-ratio control means for controlling the excess air ratio of the burner to a specified excess air ratio, wherein the excess-air-ratio control means includes outside-air temperature detection means and controls the excess air ratio to the specified excess air ratio based on a detection signal derived from the outside-air temperature detection means.
In one embodiment, there is provided a combustion apparatus for NOx reduction as described above, wherein the excess-air-ratio control means includes combustion-use-air flow rate adjusting means provided on an air supply passage and serving for feeding combustion-use air to the burner, and the combustion-use-air flow rate adjusting means controls an opening of the combustion-use-air flow rate adjusting means based on a detection signal derived from the outside-air temperature detection means, thereby fulfilling the control to the specified excess air ratio.
In one embodiment, there is provided a combustion apparatus for NOx reduction as described above, wherein the combustion-use-air flow rate adjusting means includes: a damper; positioning means for determining rotational position of the damper; and fine adjustment means for acting on the positioning means to finely adjust the rotational position of the damper in response to a detected temperature of the outside-air temperature detection means.
In one embodiment, there is provided a combustion apparatus for NOx reduction as described above, wherein the excess-air-ratio control means controls rotational speed of a blower, which feeds combustion-use air to the burner, based on a detection signal derived from the outside-air temperature detection means, thereby fulfilling the control to the specified excess air ratio.
Further, aspects of the present invention will be described according to the embodiments. Before the description of the embodiments, terms used herein and the drawings are explained. The combustion gas includes burning-reaction ongoing (under-combustion-process) combustion gas, and combustion gas that has completed burning reaction. Then, the burning-reaction ongoing gas refers to combustion gas that is under burning reaction, and the burning-completed gas refers to combustion gas that has completely burning-reacted. The burning-reaction ongoing gas is indeed a concept of substance, but can also be referred to as flame as a concept of state because it generally includes a visible flame so as to be in a flame state. Therefore, herein, the burning-reaction ongoing gas is referred to also as flame or burning flame from time to time. Further, the exhaust gas (flue gas) refers to burning-completed gas that has decreased in temperature under an effect of endothermic action by heat transfer tubes or the like.
Also, the combustion gas temperature, unless otherwise specified, means the temperature of burning-reaction ongoing gas, equivalent to combustion temperature or combustion flame temperature. Further, the suppression of combustion gas temperature refers to suppressing the maximum value of combustion gas (combustion flame) temperature to a low one. In addition, normally, burning reaction is continuing although in a trace amount even in the burning-completed gas, and so the combustion completion does not mean a 100% completion of burning reaction.
Further, the excess air ratio, which is expressed as (actual amount of combustion air)/(theoretical amount of combustion air), corresponds in a specified relationship to exhaust-gas O2(%) (oxygen concentration in exhaust gas), therefore being expressed in exhaust-gas O2(%). Also, the value of NOx shows a value at 0% O2 in the exhaust gas, dry basis, while the value of CO shows not an equivalent value but a reading value.
Next, as a detailed description of the foregoing characteristics of the present invention, embodiments of the present invention are described. The present invention is applied to thermal equipment (or combustion equipment) such as small-size once-through boilers or other water-tube boilers, water heaters, reheaters of absorption refrigerators or the like. The thermal equipment has a burner and a group of heat absorbers to be heated by combustion gas derived from the burner.
An embodiment of the apparatus according to the present invention is a NOx reduction combustion apparatus for fulfilling NOx reduction by controlling temperature of combustion gas derived from a burner, comprising: NOx reduction means having an excess air ratio versus NOx characteristic that generated NOx value decreases with increasing excess air ratio of the burner, and an excess air ratio versus CO characteristic that exhaust CO value increases with increasing excess air ratio; and excess-air-ratio control means for controlling the excess air ratio of the burner to a specified excess air ratio, wherein the excess-air-ratio control means includes outside-air temperature detection means and controls the excess air ratio to the specified excess air ratio based on a detection signal derived from the outside-air temperature detection means.
The specified high excess air ratio is determined in the following manner. Given a NOx reduction target value of 10 ppm, an excess air ratio corresponding to the target value is determined under the condition of the excess air ratio versus NOx characteristic of the NOx reduction means, and then the excess air ratio determined in this way or a value higher than the excess air ratio is taken as a specified high excess air ratio. Finally, the specified high excess air ratio corresponds to the NOx reduction target value.
The outside-air temperature detection means detects outside-air temperature, i.e., room temperature of a room in which thermal equipment is installed. By this outside-air temperature detection means, combustion-use-air flow rate adjusting means such as a blower or a damper provided on the air inlet passage between the blower and the burner is controlled so that the excess air ratio is controlled to a constant one.
According to the excess air ratio control by this outside-air temperature detection means, since the outside-air temperature that is the greatest factor of variations in excess air ratio of thermal equipment, i.e., variations in generated NOx value is directly captured and controlled, stable control of NOx value can be achieved regardless of simple control constitution. Also, the exhaust CO value derived from the NOx reduction means can also be controlled to a constant one.
Further, the excess-air-ratio control means may also be so designed that the excess air ratio is controlled to a specified one by controlling the opening of a combustion-use-air flow rate adjusting means such as a damper, valve or the like provided in the upstream of the blower, for example, on the air inlet passage of the blower, in another embodiment.
Preferably, the combustion-use-air flow rate adjusting means includes a damper, positioning means for determining rotational position of the damper, and fine adjustment means for acting on the positioning means to finely adjust the rotational position of the damper in response to a detected temperature of the outside-air temperature detection means.
The NOx reduction means is implemented, preferably, by a NOx reduction means that makes the generated NOx value not more than 10 ppm. This NOx reduction means has an excess air ratio versus NOx characteristic that the generated NOx value decreases with increasing excess air ratio of the burner, where the generated NOx value decreases to not more than 10 ppm at not less than a specified excess air ratio, as well as an excess air ratio versus CO characteristic that the exhaust CO value increases with increasing excess air ratio. This excess air ratio versus CO characteristic has a characteristic that if the excess air ratio is set to such a value that the generated NOx value falls under 10 ppm, then the exhaust CO value abruptly increases.
A preferable mode of the NOx reduction means is that combustion gas temperature is suppressed by a combination of: a combustion-gas-temperature suppression means for doing the suppression by burning a fully-premixing type gas burner at a high excess air ratio (hereinafter, referred to as xe2x80x9cfirst suppression meansxe2x80x9d); a combustion-gas-temperature suppression means for doing the suppression by heat absorbers (hereinafter, referred to as xe2x80x9csecond suppression meansxe2x80x9d); a combustion-gas-temperature suppression means for doing the suppression by recirculating burning-completed gas to a burning reaction zone (hereinafter, referred to as xe2x80x9cthird suppression meansxe2x80x9d); and a combustion-gas-temperature suppression means for doing the suppression by addition of water or addition of steam (hereinafter, referred to as xe2x80x9cwater/steam additionxe2x80x9d) to the burning reaction zone (hereinafter, referred to as xe2x80x9cfourth suppression meansxe2x80x9d). The burning reaction zone refers to a zone where burning-reaction ongoing gas is present.
The first suppression means is based on the following principle. That is, when the burner is burned at a high excess air ratio, the combustion gas temperature is suppressed so that the NOx value decreases. The high excess air ratio in this case is 5% O2 or more contained in exhaust gas, preferably, not less than 5.5% O2. This suppression effect acts generally uniformly on the entire burning reaction zone formed by the burner.
The second suppression means is based on the following principle. That is, the NOx value is reduced by suppressing the combustion gas temperature by a cooling effect of heat absorbers implemented by arranging a multiplicity of heat absorbers in the burning-reaction ongoing gas derived from the burner, i.e., in the burning reaction zone. This second suppression means is implemented by arranging the heat absorbers to cool the burning-reaction ongoing gas, hence a nonuniform cooling. There are also sites where the burning is ongoing actively in the gaps between the heat absorbers of the burning reaction zone. Particularly in the downstream of the heat absorbers, eddy currents are formed so that the combustion flame is stabilized by the heat absorbers. The heat absorbers are implemented by heat transfer tubes such as water tubes, but this is not limitative.
The arrangement configuration as to how the heat absorbers are arranged with respect to the flow of the burning-reaction ongoing gas, includes the following two modes. One of those arrangement configurations is that a combustion gas passage is formed so as to allow combustion gas to flow generally linearly therethrough from the burner to the exhaust gas outlet, and moreover the heat absorbers are arranged so as to cross the burning-reaction ongoing gas derived from the burner with gaps present among the heat absorbers to allow the combustion gas to flow therethrough. The other arrangement configuration is that heat absorbers are arrayed in an annular state with gaps present thereamong to allow the combustion gas to flow therethrough, so that the combustion gas derived from the burner flows radially from the inside of the annular heat absorbers toward the heat absorbers, where the heat absorbers are arranged in the burning-reaction ongoing gas derived from the burner. The latter configuration is described in detail in U.S. Pat. No. 6,029,614, the disclosure of which is hereby incorporated by reference.
The third suppression means is what is called exhaust-gas recirculation combustion method. Exhaust gas which has decreased in temperature through endothermic action by the heat absorbers and is then to be emitted to the atmosphere is partly mixed with combustion-use air via an exhaust-gas recirculation passage. The combustion gas temperature is suppressed by a cooling effect of the mixed exhaust gas, by which NOx value is reduced. This third suppression means also exerts uniform cooling of combustion gas.
The fourth suppression means is water/steam addition to the burning reaction zone. By this water/steam addition, the burning-reaction ongoing gas is cooled, so that the combustion gas temperature is suppressed and the NOx value is reduced. This fourth suppression means also exerts uniform cooling of the combustion gas. The water/steam addition may be carried out in the exhaust-gas recirculation passage in another embodiment. Besides, in an embodiment in which the burner is provided as a fully-premixing type gas burner and mixed gas of combustion-use air and fuel gas is fed to the burner by a blower, it is possible to perform the steam addition between the burner and the blower. For the water addition, water is added in the form of mist.
Working effects by the combination of the first to fourth suppression means are as follows. Enhancing the functions of the individual suppression means singly would cause drawbacks of the respective suppression means to matter. However, combining the four suppression means makes it possible to achieve super NOx reduction relatively easily without causing the emergence of those drawbacks. In particular, later-described unstable characteristics of the fourth suppression means are alleviated, so that stable NOx reduction can be achieved.
It is noted that the functional enhancement of the first suppression means (premixing high excess-air-ratio combustion) is to increase the excess air ratio. Due to this functional enhancement, there would occur a halt of burning reaction and an unstable combustion of the combustion burner. Also, the functional enhancement of the second suppression means (heat-absorber cooling) is the provision of the heat transfer tubes in contact with the burner or the increasing of the heat-transfer-surface density of the heat absorbers. Due to this functional enhancement, there would occur an increase in pressure loss or an unstable combustion such as oscillating combustion.
Also, the functional enhancement of the third suppression means (exhaust gas recirculation) is to increase the exhaust-gas recirculation quantity. Due to this functional enhancement, there would occur an amplification of the unstable characteristics of the third suppression means. That is, the exhaust gas recirculation has a characteristic that the exhaust-gas flow rate or temperature changes with changes in combustion quantity or changes in load. An increase in the exhaust-gas recirculation quantity would cause these unstable characteristics to be amplified, making it impossible to achieve a stable super NOx reduction. Also, due to the functional enhancement of the third suppression means, burning reaction would be suppressed, causing an emission increase of CO and unburned components as well as an increase in thermal loss. Further, increasing the exhaust-gas recirculation quantity would cause the blower load to increase.
Also, the functional enhancement of the fourth suppression means (water addition/steam addition) is to increase the quantity of water to be added. Due to this functional enhancement, the quantity of condensations would increase with increasing thermal loss, where, particularly in boilers having a feed water preheater for preheating the water fed to the heat absorbers by exhaust gas, there would matter corrosion of the feed water preheater due to the condensations.
According to the preferable embodiments of the NOx reduction means as described above, since the first to fourth suppression means are combined together, the problems that would otherwise emerge upon enhancing the functions of the individual suppression means each singly can be prevented from becoming issues.
Further, the NOx reduction means includes the following five modifications: (1) a mode in which three suppression means of the second suppression means (heat-absorber cooling), the third suppression means (exhaust gas recirculation) and the fourth suppression means (water/steam addition) are combined together excluding the first suppression means (premixing high excess-air-ratio combustion); (2) a mode in which three suppression means of the first suppression means (premixing high excess-air-ratio combustion), the second suppression means (water-tube cooling) and the third suppression means (exhaust gas recirculation) are combined together; (3) a mode in which three suppression means of the first suppression means (premixing high excess-air-ratio combustion), the second suppression means (heat-absorber cooling) and the fourth suppression means (water/steam addition) are combined together; (4) a mode in which two suppression means of the second suppression means (water-tube cooling) and the third suppression means (exhaust gas recirculation) are combined together; and (5) a mode in which two suppression means of the second suppression means (heat-absorber cooling) and the fourth suppression means (water/steam addition) are combined together.